Vacuum tube



July 21, 1936. H SNOW 2,048,224

VACUUM TUBE Original Filed March 19, 1950 4 Sheets-Sheet l F4764 596a INVENTOR.

A TTORNEZS.

H. A. SNOW VACUUM TUBE July 21, 1936.

Original Filed March 19, 1930 4 SheetsSheet 2 INVENTOR M BY W W V I ATTORNEYS July 21, 1936.

H. A. SNOW 2,048,224

VACUUM TUBE Original Filed March 19, 1950 4 Sheets- Sheet 3 w w .6 find/o 519 11-20: y l a/fs, Esq 6.50/TC, Mada/022w a) 604/ 5070 Mada/0764511 IN V EN TOR.

ATTORNEYS.

July 21, 1936. A, W 2,048,224

VACUUM TUBE Original Filed March 19, 1930 4 Sheets-Sheet 4 IN V EN TOR.

A TTORNEYS.

Patented July 21, 1936 UNITED STATES PATENT OFFICE VACUUM TUBE Delaware Application March 19, 1930, Serial No. 437,225 Renewed January 2, 1936 11 Claims.

This invention relates to space discharge tubes of the type including a cathode, a control grid and an anode.

The problem of preventing modulation distortion in an amplifier stage, or in cascaded stages, has imposed severe limitations upon the range of signal voltages which may be applied to the amplifier. In radio receivers, for example, high sensitivity is desirable for the reception of weak signals, and some form of manual, or automatic, control must be provided to reduce the amplifier transmission or gain, when stronger signals are received. When a receiver of high sensitivity is operated in the vicinity of a broadcasting station, it is not unusual to find that the signal voltage applied to the first carrier wave amplifier is greater than the voltage required on the detector for normal output at the loud speaker. With the present types of electron discharge tubes, it is usual to adjust one of the operating potentials applied to the tube electrodes to decrease the amplification as the received signal strength increases.

Within the range of relatively low signal strengths, this reduction of amplification is not accompanied by modulation distortion, but with increasing signal strengths distortion is introduced when the amplification rate is adjusted to maintain an approximately constant output. Furthermore, within the range of higher signal strengths, it is frequently diflicult to adjust the amplification to maintain constant output since the transconductance of the tube changes very rapidly for small changes in the transmission control voltage. This restricts the amplification control to a small range of applied control voltages, and, unfortunately, the rate of change of amplification is more gradual in the range of high amplification where a rapid change of amplification for small changes in control voltage would be permissible.

The improvements disclosed herein in connection with an electron discharge tube to reduce distortion will also reduce a large part of the cross talk, which is that species of interference originating in radio frequency amplifier tubes by modulation between two or more signals. Cross talk effects in radio frequency amplifiers depend upon the high order curvature parameters of the tube and are to that extent related to the problem of distortion. It has been demonstrated that there is an intimate relationship between the problems of distortion and cross talk in radio frequency amplifiers and their elimination by means of variable mu tubes.

It is, therefore, one of the main objects of the present invention to provide a space discharge tube having such operating characteristics that no distortion is introduced when, for increasing signal strengths, the operating potentials are so adjusted that the amplification rate is reduced to a small fraction of the maximum amplification.

A further object of the invention is to provide an electron discharge tube having such characteristics that, when the potentials are adjusted to give a relatively low amplification of strong signals, the change in transconductance for a given change in the gain control voltage is much lower than is the case with the known types of 15 tubes.

Another, and important, object of the invention is to provide a high frequency amplifier tube capable of adjustment to give an undistorted output of approximately constant magnitude over a wide range of applied carrier voltages.

A more specific object of the invention is to provide an electronic amplifier in which different portions of the electron stream are influenced at different rates by the voltages applied to the control grid.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically several arrangements whereby my invention may be carried into effeet.

In the drawings,

Fig. 1 is a perspective view, partly in section, of a screen grid tube embodying the invention,

Figs. 2 to 7b inclusive are diagrammatic views 4( illustrating various embodiments of the invention,

Fig. 8 is a curve sheet showing the variations of plate current with grid bias for a tube such as shown in Fig. 1,

Fig. 9 is a curve sheet showing the relation between control grid voltage and transconductance for tubes embodying the invention,

Fig. 10 is a curve sheet showing the relation between permissible maximum input voltages and control grid bias voltages, and

Fig. 11 is a curve sheet showing the performance of a three stage amplifier employing the novel form of tube. v

Referring to the accompanying drawings wherein like reference characters in the different figures designate the same elements, the invention is shown in Fig. 1 as embodied in a form of tube known commercially as a screen grid tube, the latter having a separate heater for the cathode. As is well known, this particular type of tube comprises an evacuated envelope enclosing a cathode C, heated by a resistance (not shown) within the cathode, an inner grid CG, an outer grid SG, a plate, or anode P, and an outer screen S which is electrically connected to the outer grid. Except for the novel construction of the control grid CG and its novel functional relationships to the remaining elements of its tube, the several elements of the tube, and their relative physical arrangement, may be substantially the same as that employed in the present commercial tubes.

In tubes of this general type, the control grid comprises a helical winding with its turns constituting transverse conductors connected to and supported by one or more foundation supports or wires I. In this particular embodiment of the present invention, the helical winding is not continuous, as in the known constructions, but comprises two sections 2 that are separated by a distance of the order of twice the pitch of the winding. The windings of each section are of the same pitch, which may be the same as that now employed in tubes of this type. This particular physical embodiment of the invention therefore physically diifers from the known construction, of the same general physical design, by the absence of two complete circumferential turns of the control grid winding.

This particular construction results in a tube in which the control exercised upon the electron stream is not uniform over the entire extent thereof.

The control grid has been shown to be divided into two sections which are mounted with a gap between them. At low negative biases the entire cathode is operative, and the tube has about the same characteristics it would have if the gap were in place. As the grid bias increases negatively the electron current through the upper and lower parts of the control grid are cut off leaving a low-mu control through the gap. At these bias voltages the tube acts as if the upper and lower sections of the control grid were formed of solid metal, and controlled the current through the gap in the ordinary manner.

This variation in control at different parts of the tube may be effected by other control grid constructions, or by other physical arrangements of the tube elements. Instead of removing turns from a grid winding of uniform pitch, the construction shown in Fig. 2 may be employed. In this form, the control grid is a continuous winding carried by the supports I, the end portions 3 of the winding being of the same pitch, and joined by an intermediate section 4 of much longer pitch.

In Fig. 3 the cathode C takes the form of a coating, represented by the stippled section on the heater element H. The plate P and screen grid SG are of the usual construction and arrangement, but one end of the control grid CG stops short of the end of the cathode.

The diagrammatic views 40. to 4e inclusive show control grids CG in which the winding pitch is not uniform throughout the entire length of the grid.

- In Fig. 4a, the winding is of progressively varying pitch from one end to the other. The control grid of Fig. 4b has one section a of one pitch, and

a second section b of a different pitch. Fig.

a diagrammatic view showing the omitted turns arrangement which is illustrated in Fig. 1, and

Fig. 4d shows one turn omitted at each side of the central turn of the grid winding. Fig. 4e shows an arrangement similar to that of Fig. 4b. but having three sections a, b, c of different pitch.

Further modifications of the invention are shown in Figs. 5a to 5d, inclusive, and as shown in Fig. 5a, the screen grid SG may take the form of a conical, or tapered, winding of uniform pitch, the remaining tube elements being of the usual cylindrical form. Any one, or more of the tube elements may be tapered, and Figs. 5b to 5d show, respectively, a tapered control grid, a tapered cathode, and a tapered plate.

An eccentric arrangement of one, or more, elements may be employed, Fig. 6a showing the axis of the cathode C inclined to the axes of the other elements, and Fig. 6b showing the cathode C parallel to, but not coaxial with, the other elements;

It will be apparent that various combinations of the several described constructions may be included in a single tube. As shown in Fig. 7a, a tapered screen grid SG may be used with the control grid of Fig. 4b having windings of two different pitches, or as shown in Fig. 7b the screen grid SG may comprise two cylindrical sections of different diameters. To prevent the flow of an excessive plate current the larger diameter section of the screen grid is located opposite the portion of the control grid from which the windings are removed, or are of the greater pitch.

It is well known that the amplification of a vacuum tube may be regulated by adjusting the bias voltage upon the control grid, the amplification decreasing as the bias voltage becomes more negative. Curves showing the relation between plate current and grid bias, 1. e., transfer characteristics, aiford an indication of the amplification at different bias voltages, since the slope of the curve at any point is a measure of the amplification when the tube is biased for operation at that point.

In Fig. 8, the solid line curve A is the transfer characteristic for a tube such as shown in Fig. 1, and the dotted line curve B is a similar curve for a commercial screen grid tube of the same general type but having a continuous control grid winding of uniform pitch. An examination of curve A shows that, with tubes embodying the invention, a control of amplification extends over a range of control grid bias of from zero to more than 30 volts. With the known tubes, the curvature of the transfer characteristic approaches zero at a control grid bias of about 15 volts.

In other words, an increase of the grid bias above approximately 15 volts negative will not be accompanied by a decrease in amplification when the known type of tube construction is employed, but with tubes embodying the invention, the amplification may be varied with changes of control grid bias throughout a range of from zero to upwardly of 30 volts. The tubes will still pass signals, by leakage transmission when the control grid biases exceed these respective values. but control of amplification is no longer possible in regions where the transconductance curves become substantially horizontal.

The curves of Fig. 9 show the relation between control grid bias and transconductance for two tubes embodying the invention, and for a similar tube which has the usual grid construction. Curve A is the transconductance-control grid bias curve for a screen grid tube of the type shown in 4c is Fig. 1, having two turns omitted from the center of the control grid. The data. for this curve and for curve A of Fig. 8 relates to the same tube. Curve A is a similar curve for a screen grid tube in which only one turn was removed from the center of the control grid, and curve B shows the characteristic properties of the conventional type of tube having a continuous control grid winding.

An examination of these curves shows that the useful range of transmission control is considerably extended by the present invention. With the known constructions, a decrease of the transconductance from about 500 mlcromhos to the valve, about 0.8 micromho, at which leakage transmission prevents further amplification control, corresponds to a change in control grid bias of about ten volts. The corresponding ranges of control grid bias for the tubes of curves A and A are, respectively, about 30 and 60 volts.

As stated above, modulation distortion may occur when, for a given signal strength, the amplification is so adjusted as to bring the output down to a desired, or standard, level. Since such distortion is due to the curvature of the transfer characteristic it will be apparent that a tube having a curve of lower curvature can transmit, without distortion, higher voltage signals than a tube having a characteristic which exhibits a region of higher curvature. An examination of the curves of Fig. 8, will show that the maximum curvature of curve A is substantially lower than that of curve B.

Modulation distortion introduced by a tube may be determined by applying a signal having a definite and constant modulation to the input of the tube, and measuring the modulation of the output signal. For small input signals the tube introduces practically no change in modulation. When the input signal increases beyond a certain value the modulation of the output signal increases rapidly due to the curvature in the tube transfer characteristic. This increase in modulation (modulation distortion) limits the maximum input signal that may be transmitted by the tube without distortion.

The curves of Fig. 10 show the relation between control grid bias voltages and the maximum input signal voltages which produce a twenty per cent rise in modulation. The 20% rise in modulation was chosen as a standard as a matter of convenience since distortion of this magnitude may be observed by car when the modulation is within the range of audible frequencies as is the case with speech or music. The data for curves A, A, and B was obtained for the same tubes as those whose characteristic curves are identified by corresponding characters in "Fig. 9, the signal in each instance being an 850 kilocycle carrier, modulated 30% at 60 cycles.

In the case of the commercial tube, curves B of Figs. 9 and 10 show that over the range of bias voltages which control the amplification, i. e., from zero to about twelve volts negative, the maximum input signal voltage which can be transmitted with not more than 20% distortion is about volts.

When the bias is adjusted to maintain a constant output signal, the maximum input signal voltage than can be applied to the tube with less than 20% distortion is about 0.3 volt, corresponding to a control grid bias of approximately 12 volts negative. For greater signal strengths, the increases of control grid bias do not alter the amplification, but do affect. the maximum signal strength which may be transmitted with less than 20% distortion. For a single tube, signal strengths falling outside the range of volume control cannot be handled by the amplifier if a constant output is essential, but in cascaded amplifiers having two or more controlled stages, the maximum voltages, as shown by the dotted line portion of curve B, may be transmitted by the first stage when amplification control without additional distortion is provided in a subsequent stage.

A similar analysis of curve A will show that the maximum signal strength which may, with out undue distortion, be transmitted to give the desired constant output voltage is about 1.1 volts, corresponding to a control grid bias of about -38 volts. For the tube with two turns removed from the control grid, curve A shows a permissible input voltage of 3 volts, with a bias of 95 volts. Furthermore, by so restricting the control grid bias voltages that the maximum can never exceed about 28 volts negative in the case of the tube of curve A, and about 65 volts in the case of the tube of curve A, the maximum carrier voltages which may be transmitted are approximately 7 and 22 volts, respectively.

The observations have been verified by tests made with a commercial radio receiver having three radio frequency amplifier stages employing commercial screen grid vacuum tubes of the 224" type.. Measured carrier wave voltages, 850 kilocycle modulated 30% at 60 cycles, were impressed upon the first amplifier and the amplification was adjusted to maintain a constant carrier voltage on the detector which, for undistorted carrier amplification, correspond to a 60 cycle audio frequency output of 4 volts across the speaker terminals.

Modulation distortion in the amplifier is evidenced by an increase in audio frequency output voltage when the carrier component or the de- By maintaining tector input remains constant. a constant carrier voltage at the detector the demodulated audio frequency output will remain constant up to the point at which modulation distortion begins. Beyond that point, the audio frequency output will rise even though the carrier voltage across the demodulator is maintained constant as the signal strength increases.

In Fig. 11, curve D shows the relationship between audio output and carrier wave input when commercial 224 tubes were used in the amplifier. To eliminate distortion in the third radio frequency stage, only the first two stages were adjusted to control the amplification. The distortion was of the same values whether the bias on the control grid of the first tube was varied, or the bias control was extended to include both the first and second tubes. From curve D it will be noted that the modulation rise begins at a carrier input of about 0.15 volt, and reaches 20% at 0.3 signal voltage or the first tube.

Curve E was plotted from data obtained with the same radio receiver when tubes embodying the invention were substituted in the radio frequency stages. The tubes were of the type shown in Fig. 1, i. e., of standard 224 type construction except that two turns were omitted from the center of the control grid, With increasing signal strength, the detector input-was maintained constant by adjusting the control grid bias simultaneously on the three amplifier stages. It is to be noted that the carrier input across the first tube increased to 10 volts before a modulation rise was apparent, and that it reached 17 volts before the modulation rise reached 20%. The variation of control grid bias with input signal strength is shown by curve F.

These results were checked qualitatively by a listening test with ordinary broadcast (music) 5 modulation. It was found that the distortion on high modulation peaks became apparent when the input on the first amplifier was from 15 to 20 volts, becoming worse as the input was increased beyond 20 volts.

These observations of actual performance in a receiver check closely with the results plotted in Figs. 9 and 10 for single stages. By employing tubes constructed in accordance with the invention, the permissible input voltage was raised from 0.3 volt to 1'7 volts, i. e., volume control with good quality reproduction can be had with input voltages about 57 times as great as those which may be applied when the known commercial form of tube is employed.

It is to be understood that the invention is not limited to any particular type of tube, but is, in general, applicable to all tubes employed for amplification control. The physical construction of the control grid, or the geometric and structural relationships of the tube elements, are subject to wide variation so long as the control grid exercises different rates of control at different portions of the electron stream. Considered broadly, the invention provides in a single tube, the electncal equivalent of two or more, amplifier tubes operating in parallel, one tube having a relatively high ratio of plate voltage to control grid voltage (high mu), and one or more, of the remaining tubes having lower ratios of plate voltage to control grid voltage (low mu). In fact, the same distortionless amplification control and substantial reduction of cross-talk over a wide range of input voltages may be secured when two or more tubes of the described different characteristics are operated in parallel. A single tube exhibiting these characteristics will usually be more economical and convenient than the parallel tube arrangement.

Although the above discussion has been limited to a consideration of modulation distortion in radio receivers, it will be apparent that the curvature of the transfer characteristic gives rise to other forms of distortion which limit the range of continuous wave and audio frequency voltages within which a tube acts as'a substantially linear mplifier. The invention provides a means for extending the range of signal voltages which may be transmitted without distortion, the signals being either of audio or radio frequency, and if of 5 radio frequency, either continuous wave or modulated. 1

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my 60 invention is by no means limited to the particular constructions shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

65 I claim:

1. An electron discharge device comprising a cathode, a coaxial plate surrounding said cathode, and a coaxial controlgrid interposed between said cathode and said plate and comprising a plu- 70 rality of helices in axial alignment and of uniform pitch such that electrons may pass between all their turns, said helices being positioned to leave between their adjacent ends an annular opening in the periphery of the grid wider than the pitch 75 of said helices.

2. An electron discharge device comprising a cathode, a cylindrical plate coaxial with and surrounding said cathode, and a cylindrical grid of uniform diameter throughout its length mounted coaxial with said cathode and said plate and 5 having throughout its length perforations through which electrons can pass radially of said grid and which are wider near the middle of said grid than near the ends thereof.

3. An. electron discharge device comprising 10 a cathode, an anode, and a. grid surrounding said cathode, said grid comprising foundation supports and transverse conductors connecting said supports and spaced to permit electrons to pass through said grid and with some of said 15 conductors near the middle of said grid separated by an .open zone in the surface of said grid of a width greater than the spacing between other conductorsnear the ends.

4. An electron discharge device comprising a 20 cathode, an anode, and a grid surrounding said cathode, said grid comprising foundation supports and transverse conductors connecting said supports, all of said conductors being spaced to provide openings through which an electron 25 stream may flow, the spatial relation of adjoining parts of said grid to corresponding parts of said cathode differing successively by uniform increments whereby as the potential of said grid becomes increasingly negative, the flow of electrons 30 is stopped from different parts of said cathode in succession.

5. An electrostatic control electrode compris ing two coaxial helices with turns spaced to permit electrons to pass radially through said helices 35 and mounted end to end with their adjacent ends electrically connected and separated by an annular opening in the periphery of said electrode wider than the space between adjacent turns of said helices. 40

6. An electrostatic control electrode comprising foundation supports and transverse conductors connecting said supports and spaced throughout the length of saidgrid that electrons may pass between said conductors and of a greater 45 spacing near, the middle than nearthe ends of said grid.

'l. An electrostatic control electrode comprising a support rod, and a grid wire wound into a helix of uniform pitch that electrons may pass 9 between the turns of said helix and having its i turns secured to said support rod with one or more turns omitted from said helix near the middle thereof to leave'an annular opening in the periphery of said electrode.

8. A grid structure for space discharge devices comprising a support rod, two helices with turns secured to said support rod and spaced to permit an electron discharge to pass radially of said helices and between said turns, said helices being mounted end to end with their adjacent ends s parated to leave in the periphery of said grid a annular opening wider than the space between turns of said helices.

9. A grid structure for space discharge devices comprising a support rod, two coaxial grid helices mounted end-to end on said rod with .the grid turns spaced to permit an electron discharge to pass radially between said turns, said helices. being mounted coaxially and end to end with their adjacent ends separated by a gap, and a grid turn positioned on said support rod and in said gap to be intermediate and spaced awa from the adjacent ends of said helices.

10. An electron discharge device comprising an electron emitting cathode, a tubular anode surrounding and coaxial with said cathode with its inner surface substantially parallel to and uniformly spaced from said cathode throughout its length, and an equi-potential tubular grid electrode interposed between and uniformly spaced throughout its length from said cathode and from said anode and comprising foundation supports and transverse conductors connecting said supports and spaced throughout the length of said grid that electron flow from said cathode to said anode may pass between said conductors and of a greater spacing near the middle than near the ends of said grid.

11. An electron discharge tube comprising a cathode, an anode, and a control grid interposed between said cathode and said anode, said grid comprising foundation supports and transverse conductors connecting said supports and spaced to permit an electron flow from said cathode through said grid to said anode, the spacing of said conductors differing along the length of said grid by increments which cause the transconductance of said tube to vary in substantially exponential relation to the voltage with reference to said cathode applied to said grid over the major part of the range of grid bias voltage and thereby cause the anode current of the tube to vary in substantially exponential relation with the voltage applied to said control grid.

HAROLD A. SNOW. 

